1. Field of the Invention
The present invention relates generally to seats for ball valves, and more particularly, but not by way of limitation, to an improved seat assembly for a trunnion mounted ball valve wherein the seat assembly is capable of adapting to varying working pressures.
2. Brief Description of Related Art
In the typical construction of a trunnion mounted ball valve, the ball is machined to provide “trunnions” that are mounted in bearings. The bearing-trunnion combination is intended to support the ball in a stationary position relative to the flow path, but allow rotation of the ball. The ball engages a pair of seat assemblies to form a seal around the ball.
The seat assemblies typically include an annular seat carrier and a ring-shaped seat positioned in a groove formed in the seat carrier. In a trunnion ball valve, the seat assemblies are free to move and respond to the internal line pressure because of differential surface area. That is, the surface area of the seat assembly being acted on by line pressure is greater on the end of the seat assembly positioned away from the ball valve than it is on the end engaging the ball. Consequently, the line pressure forces the seat assembly toward and against the ball valve. Both the upstream and downstream seat assemblies respond to line pressure in the same way, thus leading to the feature of a trunnion valve known as “double block and bleed.”
Another feature of a trunnion ball valve is reduced operating torque at higher working pressures. Operating torque is primarily a function of the friction created by the seat contacting the ball at the sealing interface. The design considerations that affect the amount of friction generally are: (1) the axial force of the seat against the ball; (2) the contact area of the seat on the ball, combined with the hardness or compressive strength of the seat; and (3) the surface finish on the ball.
Traditional design efforts have been directed at minimizing the operating torque for opening and closing the valve at maximum working pressure. To achieve this desired result, it has generally been necessary to sacrifice low pressure sealability. As a result, trunnion mounted valves have notoriously poor performance when sealing at working pressures much less than maximum rated pressure. To produce the lowest torque at the most critical point (maximum rated working pressure), one would choose a seat with a thin seal surface to reduce contact area, a hard sealing material to reduce friction, and a small differential area to reduce the axial force. The low pressure sealability of such a design would be poor. To improve sealing at reduced pressures, one would need to change to a softer sealing material and then add springs to create a preload of axial force. In each case, the seal is not optimized except within a narrow range of pressures.
To this end, a need exists for an improved seat assembly that is capable of adapting to varying working pressures. It is to such an improved seat assembly that the present invention is directed.